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-: Computer Viruses :-

What is
a Computer Virus ?
A potentially damaging computer programme capable of reproducing itself
causing great harm to files or other programs without permission or
knowledge of the user.

Types of viruses :-
The different types of viruses are as follows-

1) Boot Sector Virus :-
Boot sector viruses infect either the master boot record of the
hard disk or the floppy drive. The boot record program responsible
for the booting of operating system is replaced by the virus. The
virus either copies the master boot program to another part of the
hard disk or overwrites it. They infect a computer when it boots
up or when it accesses the infected floppy disk in the floppy drive.
i.e. Once a system is infected with a boot-sector virus, any non-write-protected
disk accessed by this system will become infected.

Examples of boot- sector viruses are Michelangelo and Stoned.

2) File or Program Viruses :-
Some files/programs, when executed, load the virus in the memory
and perform predefined functions to infect the system. They infect
program files with extensions like .EXE, .COM, .BIN, .DRV and .SYS
.

Some common file viruses are Sunday, Cascade.

3) Multipartite Viruses :-
A multipartite virus is a computer virus that infects multiple different
target platforms, and remains recursively infective in each target.
It attempts to attack both the boot sector and the executable, or
programs, files at the same time. When the virus attaches to the
boot sector, it will in turn affect the system’s files, and
when the virus attaches to the files, it will in turn infect the
boot sector.
This type of virus can re-infect a system over and over again if
all parts of the virus are not eradicated.

Ghostball was the first multipartite virus, discovered by Fridrik
Skulason in October 1989.
Other examples are Invader, Flip, etc.

4) Stealth Viruses :-
These viruses are stealthy in nature means it uses various methods
for hiding themselves to avoid detection. They sometimes remove
themselves from the memory temporarily to avoid detection by antivirus.
They are somewhat difficult to detect. When an antivirus program
tries to detect the virus, the stealth virus feeds the antivirus
program a clean image of the file or boot sector.

5) Polymorphic Viruses :-
Polymorphic viruses have the ability to mutate implying that they
change the viral code known as the signature each time they spread
or infect. Thus an antivirus program which is scanning for specific
virus codes unable to detect it's presense.

6) Macro Viruses :-
A macro virus is a computer virus that "infects" a Microsoft
Word or similar application and causes a sequence of actions to
be performed automatically when the application is started or something
else triggers it. Macro viruses tend to be surprising but relatively
harmless.A macro virus is often spread as an e-mail virus. Well-known
examples are Concept Virus and Melissa Worm.